(1) Field of the Invention
The present invention is directed to low frequency acoustic transmitters, and more specifically to a miniature lightweight transmitter that mechanically generates low-frequency acoustic energy by using one or more resonant gas bubbles.
(2) Description of the Prior Art
Generating coherent acoustic energy by mechanical means leads to very large systems when low frequencies are required. For example, a resonant projector system has a natural frequency of √{square root over (k/m)} where k and m are the effective mechanical stiffness and mass, respectively, of the resonant projector system. Decreasing k and increasing m both tend to increase dimensions. Non-resonant systems (i.e., moving coil projectors) also increase in size and weight as the required upper frequency is decreased to maintain a given sound pressure level (SPL). A system can be as small as needed if there are no SPL requirements. What is needed is a mechanical means of generating coherent acoustic energy at low frequencies contained by a small system. Such a system would ideally use resonant gas bubbles such as air bubbles in a fluid such as water. A resonant gas bubble resonates very effectively at low frequencies with small dimensions. The effective stiffness of a resonant air bubble for example is governed by that of air, while its effective mass is that of the added mass of the surrounding water. For example, a 1 cm radius bubble resonates at approximately 300 Hz. In general the formula for the resonant frequency is as follows:
                              f          r                =                              1                          2              ⁢                                                          ⁢              π              ⁢                                                          ⁢              a                                ⁢                                                    3                ⁢                                                                  ⁢                γ                ⁢                                                                  ⁢                                  P                  0                                                            ρ                0                                                                        (        1        )            Here P0 is the hydrostatic pressure, ρ0 is the density of water, and γ is the ratio of specific heats of the gas (i.e., air), and a is the bubble radius. The bubble frequency response is narrow band in nature, requiring a plurality of different sized bubbles if a broadband waveform is required.